Heavy oils, such as Californian Kern River crude oil, Venezuelan Hamaca crude oil or other heavy oils, are typically dense (i.e. API gravity below about 25) and have a high viscosity (i.e. a SUS at 100.degree. F. greater than 1000) Because of these properties, transportation of heavy oil from the well head to the refinery by pipeline is more difficult and expensive than transporting lighter crude oils. The same is true for the pipeline transportation and pumping of other viscous hydrocarbons generated during the refining process such as atmospheric residuals, vacuum flash drum bottoms, bitumen, deasphalter bottoms, tars, etc. The transportation of heavy oil and other heavy hydrocarbons by pipeline requires that the viscosity be low enough so that the size of the pipeline and the pumping requirements are economically optimum.
There are several methods known to one of skill in the art by which heavy oil and heavy hydrocarbons, (hereafter collectively referred to as heavy oil) may be transported by pipeline. These methods include: heating; dilution; oil/water emulsion formation; core annular flow; and partial field upgrading. Each method, however, has its own strengths and weaknesses as discussed below.
Heating is a common method utilized to overcome the above noted problems of transporting heavy oil by pipeline. The basis for this method lies in the fact that as heavy oil is heated the viscosity of the heavy oil is reduced and thus made easier to pump. Thus it is important to heat the oil to a point where the oil has a substantially reduced viscosity. Typically that temperature may be greater than 100.degree. F. (38.degree. C.) and typically may be 200.degree. F. (93.degree. C.) or more depending upon the properties of the heavy oil. In order to retain the heat imparted to the heavy oil, thus maintaining the ability to pump the heavy oil, insulated pipelines are utilized to minimize heat loss. In addition, periodic heating stations, in addition the pump stations may be utilized. A principle draw back to the use of heated pipelines is the high capital and operational cost of operating such a heated pipeline over long distances. In addition, underwater pipeline transportation of heavy oil through a heated pipeline is very difficult due to the cooling effect of the surrounding water and the practical difficulty of maintaining pumping stations and heating stations.
An alternative to heating is dilution of the heavy oil with a less viscous hydrocarbon such as condensate, natural gasoline or naphtha. This method is based on the principle that the addition of from about 10% vol. to about 50% volume of the less viscous hydrocarbons reduces the viscosity of the heavy oil by dilution, thus making pipeline transportation possible. However, the use of this method on a large scale can be cost prohibitive due to the need for mixing and separating stations at each end of the pipeline as well as a return pipeline so that less viscous hydrocarbon, once separated from the heavy oil at the destination, can be reutilized in the transportation of heavy oil. further the use of a diluent reduces the capacity of the pipeline to transport heavy oil.
Another method utilized in the transportation of heavy oil is the formation of the heavy oil into an oil-in-water emulsion. In such a method the heavy oil is suspended as micro-spheres of oil stabilized in a water continuous phase by the use of surfactants and detergents thus achieving a reduction in the apparent viscosity. One of skill in the art should understand that as the water content of the heavy oil/water emulsion increases, the viscosity decreases exponentially. The principle difficulty with the use of this technology is the selection and cost of the surfactant component of the emulsion. Not only must the surfactant be capable of stabilizing the emulsion during transportation, but it also must be capable of separation once the destination point of the pipeline is reached. Further complicating the situation is that often large volumes of water (i.e. greater than about 50%) are needed to reduce the viscosity to suitable levels. Thus the total possible capacity of the pipeline is not used in the transportation of oil thereby impacting the economics of the operation of the pipeline. Illustrative descriptions of the above described oil-in-water emulsions systems may be found for example in U.S. Pat. Nos. 4,190,069; 4,993,448, 4,857,621 and 5,021,526.
Transportation of the oil using core annular flow is another proposed concept. Here an artificially created film of water surrounds the oil core concentrically. This reduces the viscosity and pressure drop almost to that which would be expected for water. These processes require that, where field emulsions are produced, these emulsions be broken first. Water, and in the case of emulsion transport, surfactants, are then added and mixed under controlled conditions to obtain a stable emulsion or core flow. Thus, the water functions as a lubricating layer between the outer annulus of the inner wall of the pipeline and the inner core of heavy oil that is being transported. There exist several difficulties with the use of annular-core flow including an easily upset flow stability, the start-up and shut down process is difficult and the pressure drop may not be sufficient to transport some heavy oils. Further as in all cases where diluents or water are used, a significant part of the capacity of the pipeline is being taken up by a non-heavy oil component, significantly adding to the cost of the system. In the case of water, it might also create a disposal problem at the receiving end of the pipeline. An illustrative description of the above described method may be found for example in U.S. Pat. No. 4,753,261.
Yet another method of reducing the viscosity of heavy oil, thus enabling it to be transported by pipeline, is partial upgrading at the well head. The goal of partial upgrading is to reduce the viscosity and increase the API gravity of the heavy oil prior to transportation. Often this is accomplished by thermal treatment with or without a catalyst in the field in moderately scaled reactors. The primary concern with such an operation is the high capital and operating costs of providing facilities in the field and the safe operation of the upgrading unit at the well site.
When pipeline transporting less viscous oils, commercial drag reduction agents may be used to reduce the pipeline/oil drag. Such agents may provide pressure drop reductions of 15-25%. One of skill in the art should appreciate that such drops in drag reduction have a significant impact on the cost of transporting the oil by pipeline. Unfortunately, commercial drag reducing agents are reported as not showing the same effect when pipeline transporting heavy oils such as Kern River or Hamaca heavy oils. One of skill in the art should also appreciate that commercial drag reducers are degraded by shearing forces in the pipeline. Therefore, commercial drag reducers are typically reinjected after each pump station in order maximize their beneficial effect.
In view of the above, there exists a continuing need for methods which reduce the pipeline drag of heavy oil.